Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression system are also commonly used in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage areas in commercial establishments. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodal.
Unlike conventional refrigeration systems used in stationary applications, transport refrigeration systems, such as those used in trucks/trailers and refrigerated containers, operate in very unpredictable and changing environments. In operation, transport refrigeration units may be exposed to ambient temperature and humidity conditions that vary widely over the course of a day and over different seasons throughout the year and in different geographical locations and climates. Also, the nature of the product stored in the temperature controlled cargo space associated with the refrigeration system may vary dramatically and unpredictably. For example, on one trip, the product being shipped in the cargo space may be deep frozen seafood, and on the next trip, the product being shipped in the cargo space may be bananas, or other fruits and vegetables freshly picked.
Generally, products loaded into the cargo space of a container or trailer from a warehouse have already been cooled to the desired product storage temperature within a refrigeration facility at the warehouse. When the chilled or frozen products have been transferred into the cargo space, the container or trailer doors are closed and the refrigeration unit is operated in a “pull down” mode to rapidly reduce the air temperature within the cargo space from ambient or near ambient temperature down to the desired product storage temperature for transport. In conventional practice, the refrigeration unit is typically designed with a refrigeration capacity sized to provide stable temperature at a low box temperature desired for a frozen product and even a deep frozen product, which would be at least as low as 0° C. (32° F.) and as low as −18° C. (about 0° F.) for a deep frozen product.
However, when product which has not been pre-cooled, such as for example fresh fruits and vegetables direct from the field, the product is loaded into the cargo space of the container or trailer “hot”, for example at ambient outdoor temperature, a substantial refrigeration load is imposed upon the transport refrigeration system operating in the pull down mode. The refrigeration system must not only reduce the temperature of the air within the cargo space, but also to reduce the temperature of the product from ambient outdoor temperature down to the desired product storage temperature. Products loaded “hot” require the refrigeration unit to pull the product temperature down to set point temperature, typically about 2° C. (about 36° F.) from product temperatures ranging as high as 38° C. (100° F.) or above.
During pull down, the refrigerant system requires high refrigerant mass flow to produce the required cooling capacity to cool the cargo space and the product down as rapidly as possible. Thus, the refrigerant compressor must operate at or near its maximum load capacity. However, the refrigeration system must also be capable of operating efficiently over a wide range of lower cooling capacities for maintaining the steady-state temperature within the cargo space within a relatively narrow range of the desired product storage temperature despite changing ambient conditions. The desired product storage temperature will vary depending upon the product being shipped. For example, in the case of bananas, the temperature within the cargo space would be held at a relatively constant temperature of 14° C. (57° F.). In such an application, the refrigeration system operates at a very low capacity to match cooling demand, which means that the compressor will operate at very low refrigerant mass flow rates as compared to the refrigerant mass flow rates at which the compressor operates during pull down.